19th Century "cross written" letters. Cross writing was a technique to save paper when paper was scarce. Every scrap mattered at one time (one of these is dated 1823) so the writer, upon reaching the end of the page, would turn the paper 90 degrees and add a second layer of text. Once it becomes familiar, the mind adapts easily and cross written letters are surprisingly legible. Charles Darwin famously used the technique.
Original Post from Dull Tool Dim Bulb the Daily Blog

Two Early 19th Century Cross Writing letters, Collection Jim Linderman